We propose to study the evolution of estrogen receptor status in two systems; human mammary cancers before and after therapy (endocrine therapy and/or chemotherapy), and GR mammary tumor before and after serial transplantation. Currently available laboratory techniques (such as for cytosol and nuclear estrogen receptors, progesterone receptors and fluorescent estrogen-conjugate stain), will be used to assess the alterations in cellular population and cellular function. By examining the changes of estrogen receptor status at the cellular level, one would be able to determine which cellular population is being affected by endocrine therapy, chemotherapy or a combination of endocrine-chemotherapy. This will lead to the development of a more effective therapeutic program for long-lasting tumor regression.